‘Current’ extensions are usually ‘signed’, and once installed using the Extension Manager > Install Extension button, they should work thereafter.
Older extensions [like some of mine at SketchUcation] are signed but ‘out of date’ because changes to newer SketchUp versions can cause that.
It does not mean they won’t work in newer SketchUp versions.
Provided that you only install extensions’ RBZ files which you had downloaded from trusted reputable sites like EWH, SketchUcation or Smustard [and several known developers own web-sites], then you can be confident in their ‘safety’ even when they say the signature is out of date: to use them simply reset your Extension Manager > Loading policy to ‘Unrestricted’.
When SketchUp restarts they’ll load.
If you get an RBZ from an untrustworthy site and/or it reports in the Extension Manager that the signature is ‘invalid’, then you might be wise to avoid it, and uninstall it…
Having a fully signed extension is actually an illusory security.
It means it meets certain coding regimes, and has been processed by the EWH or the SketchUp-portal [i.e. the non-EWH RBZ files like mine and Fredo’s], if its signing is ‘out-of-date’ it has not yet been updated for the newest SketchUp version; if it’s ‘invalid’ something broke the signature - i.e. it might have been hacked !
I have made hundreds of extensions/plugins and where they need to be updated for compatibility with newer SketchUp versions they will have been, but if they still work without an update then chances are the signature is reported as ‘out-of-date’, some very old ones might have never have been signed !
SketchUp/Trimble failed to appreciate the admin-overhead that the EWH and the portal-signing process places on prolific authors - especially when their RBZ files are available ‘for free’.