MartinS said

Is Quicktext compatible with this new new version? If not, is there a similar alternative? Or will compatibility return with later versions?

I don't know. Why don't you have a look on the addon site and see what versions it works with. https://addons.thunderbird.net/en-US/thunderbird/addon/quicktext/?src=ss

Thanks. Actually it transpires that it is compatible, but for some reason TB doesn't recognise the compatibility (and brings up an incompatibility message), but updating the add-on manually has restored it.