There've been a few, but the first one that impacted me was Chrome's switch to requiring the SameSite=None flag on Set-Cookie headers[1] in a third party contex

There've been a few, but the first one that impacted me was Chrome's switch to requiring the SameSite=None flag on Set-Cookie headers[1] in a third party context. > Warning: A number of older versions of browsers including Chrome, Safari, and UC browser are incompatible with the new None attribute and may ignore or restrict the cookie. This caveat meant that UA sniffing was the only reliable means of setting cookies that were guaranteed to work properly. 1: https://web.dev/articles/samesite-cookies-explained https://web.dev/articles/samesite-cookies-explained