One breaking change, that they doesn't list as breaking (I guess since I assume the old was always broken) is:
Dynamic registration of LSP capabilities. An implication of this change is that checking a client's server_capabilities is no longer a sufficient indicator to see if a server supports a feature. Instead use client.supports_method(<method>). It considers both the dynamic capabilities and static server_capabilities.
So if you had code like
if client.server_capabilities.inlayHintProvider then
...
end
you now should use
if client.supports_method("inlayHintProvider") then
...
end
So, not really a breaking change I guess, but something you should change any way.