What do we know about the ultimate fate of the Borg?

There is that one disabled Borg cube in Star Trek: Picard, which the Romulan Free State was studying (as of 2399). Maybe they figured out how to replicate the process that disabled it, and then used that to wipe the Borg out.

We haven’t yet heard a mention of the Borg in the 32nd century in Star Trek: Discovery, although transwarp conduits (which the Borg used back in the 24th century) were still in existence, albeit apparently littered with ship debris.

If all Borg ships had dilithium on board, and if the Borg lived exclusively on their ships, it’s possible they wiped out by the Burn. It’s hard to imagine a Borg cube being that vulnerable to dilithium explosion though.

Taking into account Janeway's actions in Voyager's "Unimatrix Zero" and the other Janeway in the "Endgame", it is possible that Borg has been destroyed as a cohesive "nation". It is unclear if the Unicomplex's destruction was caused by the neurolytic pathogen or was a quarantine attempt of sorts, but USS Voyager also destroyed the transwarp network which was another huge factor of the Borg superiority. It is unclear how much of a network, though. It all depends on the interpretation, of which one possibility is 100%, as it was The Queen's responsibility to maintain said network. But then again, in 2373 (technically 2063) Data destroyed Queen the first time, without, seemingly, much impact on the Collective.

In addition First Contact proves definitely that Starfleet has the means to effectively resist the Borg to some extent, so expectation is Alpha Quadrant will survive the Collective just fine.

As mentioned, I'm not sure that The Borg Collective exists anymore, but there are definitely cells of the Borg "species" here and there, consisting of beings freed from the Collective before the Unicomplex's destruction, scattered across all four (definitely three, not sure of Beta, but I think Borg had a transwarp coundit terminal there, too) Quadrants. It's possible that they, along with The Voth (as pointed out by Valorum), Species 8472 and possibly equivalents of Species 116 (sufficiently advanced to resist Borg on one-on-one basis) will successfully resist The Collective. After all, USS Voyager surveyed less than 10% of Delta Quarant, so assumption of above is more than reasonable.

Star Trek: Destiny is a trilogy by David Mack released in 2008.

Set in February 2381, the trilogy is a crossover event featuring characters from Star Trek: The Next Generation, Star Trek: Deep Space Nine, Star Trek: Voyager, Star Trek: Titan and, as announced by Marco Palmieri, "elements from Trek's past, coming together in an ambitious tale that will shape the future of the Star Trek universe."

The trilogy deals with both the birth and end of the Borg with their ultimate fate being;