InderalokaMC SMP is a crossplay SMP server for both Java and Bedrock players, with support from 1.7 all the way up to 1.21.8. It runs in online mode false, uses SRV, and mixes in awesome datapacks and plugins to keep the server feeling fresh without losing that SMP vibe.
The server is set up around survival, economy, and roleplay, so there is room to build, play, trade, and just hang out with the community. It is advertised as lag free, with 24/7 support and a stable setup for players who want a chill server that is still active and maintained.
If you are looking for a vanilla style SMP with a bit more going on under the hood, InderalokaMC is worth checking out. Java and Bedrock are both supported, so friends on different platforms can join in together.