Check their site:
https://help.sketchup.com/en/end-of-classic-license-faq
A perpetual license will work as long as your system can run it (on Windows, probably 50+ years, on Mac 1 if you keep updating your OS)
If you trade in your classic now (before the end date of 4 november 2020) you get to keep the version that is currently active and have a discount for 1 or two year subscription.
Your ‘migrated’ version stays perpetually active.
You can still buy maintenance (order before 4 November 2020!) with additional months to the definite end date that you can have the classic ‘Maintenance & Support’ plan (3 november 2021)
After that, it keeps working (it’s perpetual!) but you are no longer entitled for support.
Maybe @TheOnlyAaron and @TheGuz can mention it in the last fireside chat?(4 November 2020)