For the record, in theory, you could try this against opposing Stealth Rock leads. While it is risky, it is still a firm possibility that you could explode as your opponent sets up Stealth Rock, use Custap Berry, and explode again. The issue with this idea is that most common Stealth Rock leads resist Explosion and would thus be hit much harder with Earthquake anyways.
The same goes for Electrode with a Sash, should this theory extend to Focus Sash as well. Sure, it could Explode against a lot of things, but it would basically have to OHKO the opposing Pokemon with Explosion and then hope to do the same again. I would imagine that this idea would bring you down to your Sash/Sturdy, so you couldn't just keep clicking Explosion over and over again.
What if Explosion had its old mechanics back?
For those of you who haven't played the old generations, Explosion used to half the opponent's defense in the damage calcuations, making it a devistating attack even if the opponent resisted it. A lot of Pokemon would be enjoy having a boosted Explosion to put a dent into any wall. These include Golem, Garbodor, Lickilicky, Skuntank, Camerupt, Exeggutor, Regirock, Muk, Drifblim, or even Haunter. Walls would have to be more wary of random things blowing up on them. The first two Pokemon I listed would probably benefit the most from this because they could set up quick hazards and then take something out with them.
I feel like the metagame would be pretty different if Explosion returned to its old mechanics. Pokemon such as Golem would be much more common-place on NU teams due to the sheer power that Explosion would once again have. For example, take a look at this calc.
252Atk Golem (+Atk) Explosion vs 252HP/0Def Alomomola (-Def): 79% - 93% (423 - 498 HP). Guaranteed 2HKO.
This was the best I could do for the purpose of this theorymon as Alomomola obviously wasn't around for DPP for me to use a DPP damage calc. Alomomola with 0 Def EV, 0 Def IVs, and a negative nature still has 4 more base defense than half of Max Def with a positive nature, but this is close enough.
With Stealth Rock on the field or a bit of previous damage, Golem's Explosion would be able to take down an Alomomola. This is obviously huge, and I would love to knock out Alomomola with such ease.
However, in spite of this, I don't feel like many of the Pokemon that are commonly used right now in the metagame would suffer too much from a reversion of Explosion's mechanics. I do think that the metagame would be effected though.
To begin with, Pokemon like Metang and Regirock would likely rise in usage due to their high Defense stats and resistance to Stealth Rock. Right now, they are great Pokemon to have on a team as is because they are good checks to common offensive threats like Jynx and Swellow. However, a return of DPP's Explosion mechanics would likely cause their usage to raise drastically.
The Pokemon that I feel would benefit the most from this would be Garbodor. Right now, the main niches of Scolipede as a Spikes user in the tier is its fantastic Speed and offensive capabilities. If Garbodor could run DPP Explosion in its arsenal, it would cause a lot more switches, giving it more chances to set up Spikes. Because of this, it would definitely allow Garbodor to rival Scolipede as an offensive Spikes stacker.
Overall, I do think the old Explosion mechanics would shake up the metagame, and I don't think that they would break it. I would love to see Explosion mechanics return, if only to see what the metagame would really look like.