Some thoughts:
Pheromosa Untiered -> 5
While pheromosa isn't the fastest viable and legal mon anymore, it still has some uses over what I see as its main competition, mienshao. Fast coaching is nice with a dynamaxer like landorus or cinderace. Cinderace being a mon mienshao can't coaching before it attacks unless you don't go max speed cinderace, which is needed in my opinion. It also has other uses in icy wind for speed control, speed swap to use with something like stakataka or glastrier. It also hits hard with close combat, and can OHKO non max landorus-therian, thundurus, and other non dynamax flying types with ice beam and triple axle, respectively. The flying type examples are also walled by mienshao, which usually only runs close combat and fake out as its attacking moves. While it may not have some things that make mienshao good, like inner focus, taunt, and ally switch, I think it is a viable option in series 9.
Gothitelle 4 -> 3
While gothitelle hasn't been very common in series 9 so far, whenever I use it, it feels very strong. The fact that the opponent cannot switch, and fake out pressure, is amazing, while also supporting teammates with trick room and fake tears. Gothitelle feels especially strong with Urshifu, glastrier, and other physical mons as the opponent cannot switch to intimidate them/absorb a max move. Also with the power level down with no restricteds, it is surprisingly bulky with something like a colbur or kasib berry to take ghost or dark type attacks, respectively.
Registeel 5 -> 4
While in most aspects, registeel is outclassed by ferrothorn and celesteela , I think its access to amnesia and not having a x4 weakness to fire like ferrothron, or a regieleki and rotom form weakness, like celesteela, makes it good enough for tier 4. While it is weak to pokemon like landorus-therian, you can intimidate it unlike regieleki or torkoal. Registeel is a good end game mon, with something like rotom-wash or other pokemon to cover its fire ground and fighting weaknesses. It can also take advantage of passive set ups like follow me trick room, but can also amnesia to boost its special defense to tank hits against something like hatterene's max flare, or torkoal's eruption quite comfortably. I don't think it should go higher than tier four, as it doesn't get access to leech seed like celesteela and ferrothorn.
252+ SpA Charcoal Torkoal Eruption (150 BP) vs. +2 252 HP / 84+ SpD Registeel in Sun: 84-102 (44.9 - 54.5%) -- 4.3% chance to 2HKO after Leftovers recovery
252+ SpA Life Orb Hatterene Max Flare vs. +2 252 HP / 84+ SpD Registeel: 68-81 (36.3 - 43.3%) -- 95.4% chance to 3HKO after Leftovers recovery
The calcs are to show that if you get an amnesia special defense boost, registeel can tank fire type attacks that ferrothorn cannot.
Edit: I accidentally put the Pheromosa nomination for tier 4, when I meant to nominate it for tier 5.
Pheromosa Untiered -> 5
While pheromosa isn't the fastest viable and legal mon anymore, it still has some uses over what I see as its main competition, mienshao. Fast coaching is nice with a dynamaxer like landorus or cinderace. Cinderace being a mon mienshao can't coaching before it attacks unless you don't go max speed cinderace, which is needed in my opinion. It also has other uses in icy wind for speed control, speed swap to use with something like stakataka or glastrier. It also hits hard with close combat, and can OHKO non max landorus-therian, thundurus, and other non dynamax flying types with ice beam and triple axle, respectively. The flying type examples are also walled by mienshao, which usually only runs close combat and fake out as its attacking moves. While it may not have some things that make mienshao good, like inner focus, taunt, and ally switch, I think it is a viable option in series 9.
Gothitelle 4 -> 3
While gothitelle hasn't been very common in series 9 so far, whenever I use it, it feels very strong. The fact that the opponent cannot switch, and fake out pressure, is amazing, while also supporting teammates with trick room and fake tears. Gothitelle feels especially strong with Urshifu, glastrier, and other physical mons as the opponent cannot switch to intimidate them/absorb a max move. Also with the power level down with no restricteds, it is surprisingly bulky with something like a colbur or kasib berry to take ghost or dark type attacks, respectively.
Registeel 5 -> 4
While in most aspects, registeel is outclassed by ferrothorn and celesteela , I think its access to amnesia and not having a x4 weakness to fire like ferrothron, or a regieleki and rotom form weakness, like celesteela, makes it good enough for tier 4. While it is weak to pokemon like landorus-therian, you can intimidate it unlike regieleki or torkoal. Registeel is a good end game mon, with something like rotom-wash or other pokemon to cover its fire ground and fighting weaknesses. It can also take advantage of passive set ups like follow me trick room, but can also amnesia to boost its special defense to tank hits against something like hatterene's max flare, or torkoal's eruption quite comfortably. I don't think it should go higher than tier four, as it doesn't get access to leech seed like celesteela and ferrothorn.
252+ SpA Charcoal Torkoal Eruption (150 BP) vs. +2 252 HP / 84+ SpD Registeel in Sun: 84-102 (44.9 - 54.5%) -- 4.3% chance to 2HKO after Leftovers recovery
252+ SpA Life Orb Hatterene Max Flare vs. +2 252 HP / 84+ SpD Registeel: 68-81 (36.3 - 43.3%) -- 95.4% chance to 3HKO after Leftovers recovery
The calcs are to show that if you get an amnesia special defense boost, registeel can tank fire type attacks that ferrothorn cannot.
Edit: I accidentally put the Pheromosa nomination for tier 4, when I meant to nominate it for tier 5.