How long should a backlink be reachable?

Hi guys,
my Site recently dropped from Position 13 to 55. Keywords are not that easy, so i pumped a lot of PBN Links. I checked my Links and and saw, that i lost about 15-20 links from ~PA30 Sites from my own PBN.
This was my own mistake, unfortunately i can not recover these backlinks.
But back to topic.
I wonder, how long a Backlink should actually be online. Is it really necessary, that these links are permanent? Does Google forget to check them, after few month maybe (6 month, 1year etc.)?
I mean honestly, some sites go offline, some domains expire, some people stop blogging etc. sooner or later Backlinks will vanish.
And besides, will it be possible to rank that site again? or is there some kind of "flag" which google leaves. Because it might looks like i bought non permanent backlinks for a short period of time or something like that and we all know, google does not like that...

Anyway, would be great if someone could drop his opinion on this matter. Permanent Backlinks are sure essential but maybe theres a certain time, google wouldnt care about, if the actual backlink site is online or not.

Thanks

You only get value from backlinks while they exist. Once the links vanish the value from them vanishes when google detects the change. So links need to exist forever if you want the value from them forever.

For continuously getting benefits your link should have live for forever as long as you want to be in business. If somehow the link get expired or get deleted then google will find a 404 link pointed on your website. Research on it. You will get more ideas about the issue.

You can only benefit from a link so long as its live. And no, Google does not forget, it keeps checking often and reflecting in your rankings.