Highly unlikely that SRB2 will drop support immediately, when it does is more likely going to be due to increasing development overhead as a result of things like API and Library updates (increasing the amount of OS specific code that is required).
It would not be impossible for support to be indefinite though, because there are still instances of Doom ports being made compatible with very old hardware and OS versions that should no longer be connected to the internet at all for safety reasons. Even SRB2 has managed something playable for the Nintendo 3DS which has been unsupported by Nintendo for around 20 years now.