All Mac hardware suffers a similar fate at some point, the 5.1 had a long life because apart from the trash can there wasn't a new Mac pro for 7 years.
Err no; not because of 7 years gap between 2012 - 2019. The 5,1 got an extended support windows because it was pragmatically sold for 3 years ( 2010 until Q4 2013). that is what got it extended lifetime.
Apple put the 2009 Mac Pro on the Vintage list in 2015 ( sales of 4,1 ended in 2010 + 5 .. 2015 Vintage )
Apple has updated its vintage and obsolete products list with various older products that have not been manufactured for at least five years,...
www.macrumors.com
went obsolete two years later , but macOS basically ended on the 2015 upgrade cycle. [ Folks hacked around this by doing firmware upgrades to fake that it was a 5,1 but 4,1 was de- supported on schedule. ]
Apple put the 2010 Mac Pro on the Vintage obsolete list in 2017 ( 2012 + 5.... vintage ). OK yes, to some extent this is Apple doing a bit of hand waving because the MP 2013 wasn't a direct replacement by counting the 2012 as something different (even though share model number) . But also entirely indicative that Apple is extending no "favors" of extra long lifetime to Mac Pro just because has Mac Pro label. ( The message was clear "Your 2010 may 'happen to work as being counted as a 2012, but writing is on the wall. " ).
Apple put the 2012 Mac Pro ( same model number as 2010 ; 5,1 ) out to pasture after 10.14 Mojave ( 2018 ) December 2013 + 5 years ..... obsolete December 2018 .
Same pattern. if there was a decent replacement the 2012 might have gotten cut off from macOS 10.14 but Apple did do a slight kick the can there. Technically "last sold" date was in November and 10.14 beta came out in June 2018 and released in Sept 2018. There is some fuzzy parts to very last update because obsolete dates tend to shift around the calendar across products , but the OS updates are now all stuck on September after WWDC so largely fixed in time from last week of Q3 (late Sept) to early November time frame.
Timing wise sometimes some system get one last update on Vintage because dates don't line up well , but by time something is strictly on the obsolete list Apple has stopped doing support. Obsolete + 1 year definately dead.
How long OSX will support a 7.1 is hard to say depends how many new versions of the Mac pro are released in the future. especially if Apple decides to drop intel for there own silicon M based CPU/GPU and develops's OSX around it.
It only takes one new Mac Pro to put the 7,1 (2019) on the "countdown clock" to Vintage/Obsolete. The more important part would be whether Apple stops selling them right away. The 2018 Intel Mini is still for sale almost 1.5 years after the M1 Mini came out. If 7,1 is sold for another 1.5 years then its "countdown clock" won't start during that period.
If Apple releases a "half sized" Mac Pro as the M-series entry as a New Mac Pro and it still has zero 3rd party GPU support, then they may well keep the old Intel one around for a while. ( perhaps with a GPU card bump AMD 6x50 update. ) .
Given how 'stale' the T2 chip is at this point, there probably isn't another Intel model coming ( that might have been a plausible option if it had been done one in 2021, but now it seems late. Not impossible but it would be surprising. )
However, Apple may just immediately 'kill' the 7,1 sales even if don't have a more robust solution.
One thing is for sure only Apple know.
Apple's policies in terms of the hardware are explicitly clear. There is no "only Apple knows the incantation ... " there.
The other complete non-mystery is that Apple largely considers macOS and the Mac hardware as one unit. MacOS is licensed to the Mac Hardware you buy it on. There is no major 'de-coupling' . So if the hardware is 'de-supported' it would be entirely opposite of Apple standard practices that the OS would still be going on that pairing. [ The pre 10.6 (?) days were users could buy new macOS on optical disk or usb drive are long dead. Apple doesn't sell upgrades or 'detached' licenses. ]
Apple does have some wiggle room to drop the macOS upgrades 'early'. Whether they do that or not is debatable.