like the question posted.

Or if i buy S1 can i stil play in main lfs server.

Nothing would change.....except for people who buy S3, ofcourse.

You'd be able to play the demo the same as now, S1 users would be playing S1 content, S2, S2 content and S3 all of the above.

Of course you could. You just wouldn´t be able to go on servers with S3 cars and tracks.

@OP: Yes

Nowadays it's even better than it was originally, because the server list is now unified. Demo, S1 and S2 can all race together, the only limitation is the content. As soon as some content/features are enabled on a server that are not available to the "lower" tier, people from this lower license level obviously can't join. When S3 is released it's most likely that Demo, S1 and S2 will still work as they do now. Of course the devs could change the conditions, but I don't see that happening.

of course you could (as has been said a bunch of times now)

It would work in the same way as someone with an S1 license right now. They can play online, but only on S1 servers like CTRA Race 1.

Thx happy new year