Everybody recognizes this situation: you're looking for an extension with a certain functionality and after you've installed it all kinds of errors appear about missing functions or even worse, you only see a blank page. Further investigation shows that the extension isn't compatible with your TYPO3 version.
For a long time extensions can set the TYPO3 version(s) they are compatible with. Unfortunately not many extension authors use that setting. You could take a look at the date of publication, but even that won't be a sure sign.
You can set the minimum for a version of TYPO3 which is not supported anymore, but then the maximum must be within the supported range. You cannot set a maximum outside the range of supported versions.
I think there is a problem there. If I set it like
Then it is not accepted and says “Error 209: Extension does not have a dependency for a supported version of TYPO3. See Announcing TER cleanup process for how to fix this.” Probalby it does not like 11.999.999. But this should be allowed because I would not like to update the ext for every minor TYPO3 update…