To be explicitly clear: no, GXE does not decay. It will always stay the same, regardless of inactivity.
This is not tied to time - this is just a matter of not having enough matches for a proper Glicko-1 to be determined.
I took my information from
this post. Specifically, from this quote:
"where Elo ladders have to manually implement rating decay to discourage periods of inactivity, Glicko builds inactivity into the rating system, with RD increasing the longer a player remains inactive."
This quote implies that GXE is specifically tied to time, because your Glicko RD is tied to time and your GXE directly uses your Glicko RD in the calculation. I intentionally didn't use the word decay in my post nor did I quote the part of the original post that mentioned decay because I was aware that it wasn't decay like the one found in Elo system.
My understanding from looking around the PS github repository was that PS uses the GXE calculation as outlined in the original X-Act post, where the formula consists only of R, RD, and constants, and that PS will display a blank for GXE if your Glicko RD is > 100. However, I'll freely admit that the PS codebase is pretty opaque to me, and I don't know if their implementation of the RD calculation conforms with the original Glickman paper. Despite this,
this file (which seems to be the file that handles the calculation, but again I don't really understand the PS codebase so I'll freely admit I'm wrong) claims that it does follow along with Glickman's paper, so RD should increase with the passage of time when not playing.
Once RD increases enough, it will be over 100 and the GXE will display as a blank, but I intentionally glossed over the details because all the user wanted to know was whether GXE info would disappear after long periods of inactivity.
Additionally, the statement that the "–" only appears when not enough matches are played is confusing me. My account has ladders where I've played 66 games and received a GXE but the user aim that I used as an example in the original post has ladders with 129 games played and no GXE, and I know that I didn't reset my games played stats on my account, so the numbers should be accurate.