The Maxim Silencer was the first commercially successful firearm sound suppressor. Developed by Hiram Percy Maxim (son of Sir Hiram Stevens Maxim, who invented the Maxim machine gun) in tandem with the automobile muffler in the early 1900s, it was patented in 1909.
https://www.forgottenweapons.com/accessories/maxim-silencer/
It’s an entirely arbitrary distinction with no formal guidelines.
“Suppressor” is primarily pushed online as a kind of softer term to rebrand “silencers” from sounding less scary. In the same way that “modern sporting rifle” has been (mostly unsuccessfully) pushed as a rebranding of more scary terms for semi-auto, detachable mag rifles.
It’s all word games.
As far as I’m concerned “silencer” is perfectly acceptable. It’s the term on numerous patents, term used in the nomenclature or even names of companies in the industry, and the term used in the stamp application. If somebody tries to “correct” you, realize that “silencer” is essentially an aspirational term for the device and always has been.