macOS 15 is Sequoia.
Apple has consistently updated Safari for the current release plus the last two releases of MacOS (26.3 also supports Sonoma), so I suspect that Safari will stop being updated when MacOS is released in September-ish 2027 - presumably when macOS 28 is released.
Just woke up to see v26.3 (manually upgrade overnight since I was ready) in old 11 Pro Max. Weird. An IMAP email account password had to be re-entered when checking emails. :O
That works for me! Gives them two more cracks at liquid glass before I have to make up my mind
Useful Siri has become the PowerBook G5. It’s an embarrassment.
Curiously, Apple does not offer 18.7.5 as an update for my iPhone 12 mini and iPad Air 4, now on 18.7.2. Since I have no intentions to ‘downgrade’ my devices to 26 I feel I’m stuck between a rock and a hard place 
Thanks to Apple 
I’m in the same boat. I have 18.7.3 on my iPhone 13 mini, thanks to the beta program. I’d gladly update to a newer 18.x version, but that is not an option.
Unfortunately, some of the security fixes in 18.7.5 and 26.3 seem to be pretty important, so it may be time to move on, even if I don’t want to.
Yeah, Apple is forcing us to upgrade. I had to get a new iPhone because of iOS v26 being slow as a heavy user. :(