Can a CMS that was created in v7 - with all the "then" available blocks, such as Nested Content > then painfully upgraded to v8 - keeping all the content > be upgraded to v13 - keeping all their current content?

if your asking, can my v8 site that has been upgraded from v7 - now be upgraded to v13 and keep my content. I think the answer is yes, nothing has been totally obsoleted in v13 (so it still has the grid, nested content etc) they are now leaving in v14 (thats my understanding anyway). going from v8 to v13 might change your code (.net framework to .net core) - but the content etc can be migrated without having to change property types.

That is great news

Yes it can, I did an really old V7 version to V10 and needed a lot of changes. For instance they where using Nested Content when it still was a community package. So all that data needed to change. Also the internal links (media picker, content picker etc.) needed to change. They where still stored as integers instead of Udi's. If you need a little help send me a private message.