I've been a member/contributor/Graver for 17 years · 11 months · 26 days.
In that time, I've seen 4 distinctly different & incompatible URL link structures on their service. The IDs have been convertible. For a limited transitional periods, they had handlers to to autoforward the old URL to the new. But even if they hadn't changed, that is only 18 years.
But there is no such thing as a stable URL. The 30 years of the WWW existence would be too short a period of history for such a claim anyway.