AFAICT SKG doesn't really make a distinction between games bought with a one-time purchase and games that are subscribed to. In their FAQ, they explicitly say it would apply to MMOs too (see https://www.stopkillinggames.com/faq "What about large-scale MMORPGs? Isn't it impossible for customers to run those when servers are shut down?) although they don't spell out whether they mean exclusively games bought with a one-time fee or games that are subscription-only.

Ross from Accursed Farms said this in a video FAQ on youtube:

" Would this initiative affect subscription games? Well, that's another question that depends on what the EU says. Personally, I think it's very unlikely because that doesn't fit well with other existing consumer laws. I think the only way you could even make that argument would be that this is necessary for preservation and most governments don't seem to care about that at all. However, I don't think this is a huge loss, since only a handful of games operate that way today. So if we can give up those but then save 99% of other games, I'm willing to make that bargain. "

so it seems like they actually are suggesting that they'd like for (a law that came out of) SKG to apply to subscription games but there's an understanding that it probably won't.

Then this is essentially a formalized buggestion, and what will almost certainly happen is companies will respond back with "Won't Fix"