@Happy0,
Thank you very much Happy0, for taking the time and informing.
It is a relief for me that the puzzles will stay where they are, even as in an archive mode. Otherwise I would even think transferring them manually into some offline database somehow, even if one by one!
Btw, I am sure you guys going to do a great job with improving the tactics trainer. But I am just hoping that you people have already looked into chesstempo's tactics trainer and at least got some inspirations from it. Because it is almost the ideal tactics trainer available online one can design and think of, but unfortunately not entirely free and a little bit cumbersome. So anything close to that trainer with lichess' minimalist design would be the best ever!
This is despite I wouldn't want Richard -ChessTempo's owner- to lose his members to Lichess as he is a tremendous person and developer. But I'm just a huge believer in open source philosophy and I want Lichess to be the best at everything chess related on the internet.
Anyway, thanks again for the reply and all the best luck with your work.