Macs tend to enter Vintage mode in about 5 years and obsolete (completely unsupported) in about 7 years
The iMac Pro entered service in 2017, became vintage in 2021 and is now obsolete. The last operating system it received was Sequoia in 2024 so it received seven operating system updates beyond what it shipped with. Presumably it will receive an additional two to three years of macOS security updates so a total of 9-10 years of macOS operating system and security support.
I've no doubt that the base iMac Pro could run Tahoe given that it outperforms the base M1 in multicore and GPU so the end of operating system support can be arbitrary. I've also no doubt that the 2015 MacBook Pro, which also received seven years of macOS operating support could also handle Sonoma and Ventura.
I expect Apple Silicon to get at least seven years of macOS operating system updates because even the M1 was so capable.
What most Apple users want to know is if they will offer more than seven years of macOS system updates. And we won't know for another two years.
Edit: The 2015 MacBook Pro 15 received its last security update in September 2024 so it received a total of nine years of macOS operating system and security updates.
The 2013 Mac Pro received its last macOS security update in September 2023 so that's an example of a Mac that received a total of ten years of macOS operating system and security updates. The 2008 Mac Pro also received a total of ten years.
The longest support I could find was the 2010 Mac Pro which received its last update in autumn 2021 for 11 years of updates.
Google AI expects the iMac Pro to receive security updates until Fall 2027 which means I get another 18 months of use before I need to consider running Linux or Windows 12 on it.