The likes of lemmy instances may be either small enough to fly under the radar, or handwaved away by legislators with “don’t worry it’s not the target of this legislation”, or even be given easy access to ministerial exemption …for now (maybe).
The kicker comes in 10-15 years’ time when, say, a government’s donor inconvenienced by protests organised using a self-hosted forum then asks the government to crack down on the age verification requirements of that forum, effectively silencing it due to the requirements being too onerous for a small forum, or the userbase being unwilling to submit their IDs/faceprints/whatever.