Due to the VoLTE end-ro-end encryption from your phone (UE) to the home operator, some countries (at least Sweden) will force downgrade inroaming users to circuit switched voice (2G/3G) in order to do lawful intercept (LI).
Is this done for everyone or just people under wiretap orders? I don't see why the network wouldn't be able to downgrade selectively since phones automatically fail over and the VoLTE service could simply refuse to authenticate or whatever.
What will they do once 2G/3G gets decommissioned? Stop allowing roaming?
It's still several years. I guess they just hope many phones of today won't be in use anymore and newer ones will have better compatibility. In practice some phones won't just work anymore, like in the original article.