It gets slightly confusing because the permanent license maintenance and support emails refer to your support subscription, which is technically right, but makes it easy to get mixed up with the subscription versions of SketchUp itself.
As you are using what is now called a Classic license, your version of SketchUp continues to work even after you stop paying for maintenance and support. You bought SketchUp, perhaps for $695 (which is effetely $575 for a permanent license to that version, and $120 to cover updates and support for the following 12 months).
Any version that comes out during the time you’re under M&S becomes your active version. You can continue to use older versions that are installed, but as soon as you install a later version, you commit to that version.
Normally people will pay the $120 per year to have 12 months of support, and to get any new releases that come out in that time. If you don’t pay, then some months later you want to have support again, there is a reinstatement fee. In your case if you stopped paying for support in January you would pay $180 to get up to date on software, and to start a new 12 months of support. But, you would still only pay $180 if you didn’t need support for now, and waited until January next year. If you’re ok using 2018 you could wait that extra time and be sure of getting the 2020 or even 2021 version.
The new subscription products are a different to do things. You don’t pay the initial $695 then $120 to keep up to date. Instead you pay $299 per year, to get both support and the latest version. But, if you stop paying, like with any subscription, you can no longer use the program. You might think that after about 3 1/2 years it would be costing you more to have the subscription product than the classic license, and that would be true, but it does come with several other items that on their own would cost over $250 per year.