The V3 incompatibility is the trash folder support. If you were on a newer release (I.E 2.14), when V3 lands, the 2.14 releases will complain the sync target is newer than what the client supports and demand an upgrade to V3.
Older clients might not demand you upgrade and just attempt to operate anyway. The delete functionality might not work properly but it might be "good enough" that the general sync nature works.
But, from the perspective of the developers, that's a clear point where one version and the next aren't compatible even if the results aren't necessarily bad, when it comes to testing in the future, nobody will be testing the older releases with the newer ones because the assumption is that everyone will have updated; so it might not be a case of the app instantly cuts out for you, but it's very much a case of "use at own risk".