Is your feature request related to a problem? Please describe.
anticheats flagged and then false banned

Is this related specifically to ViaFabric?
yes, im using viafabric client mod to play older version server[mostly 1.8.x]

Describe the solution you'd like
disable swim and 1.5block sneak <1.13 protocol

Describe alternatives you've considered
X

Which versions is this for?
snapshot21w41a

This is most likely out of scope for ViaFabric. You should consider trying multiconnect instead as its built with clientside usage in mind and thus has both of the things you mentioned fixed.

You can use multiconnect as a workaround for that, although it's very likely that this is out of scope for viaversion to allow the ability to disable it entirely; but some servers like hypixel tried to fix it themselves by using their own patches instead of lagging back the player as a whole.

This is most likely out of scope for ViaFabric. You should consider trying multiconnect instead as its built with clientside usage in mind and thus has both of the things you mentioned fixed.

Multiconnect does not support latest snapshot :( [Im lazy to switch clients]

This is most likely out of scope for ViaFabric. You should consider trying multiconnect instead as its built with clientside usage in mind and thus has both of the things you mentioned fixed.

Multiconnect does not support latest snapshot :( [Im lazy to switch clients]

As for that, currently confirmed as there hasn't been an update since september afraik; but you can file an issue to earthcomputer's repository to try and help you out with it - otherwise there's little we can do to back this up.

Out of scope for ViaFabric, use https://github.com/ViaVersion/ViaFabricPlus