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(). 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.