Can try some of these ideas. https://www.wikihow.com/Watch-Deleted-YouTube-Videos-with-the-Url

i heard there are some websites for alt youtube front end that have cache feature so that if video gets deleted we can access it on other front end, is that true?

This is exactly why in my free time I’m developing a DHT (on i2p) where given a checksum users can retrieve a file.

This would allow client applications to have an index (in this case YouTube video ID zVxDWORcQBU=<checksum>) so that content is always available from peers. Invidious could theoretically use it fetch the content from other Invidious peers rather than directly from YouTube.

My suggestion from now is to find a way to save the videos yourself—I too have found myself in this situation and it’s annoying.