Although Apple do not state a time frame for how long "subscriptions" are available, when I look at mine, it seems to be about three months.
For older posts, you have to use a search engine with a site-delimited search string. Example:
site:discussions.apple.com "Abstractman23"
will return most posts you have made here that, without the time limit, would otherwise show up in Subscriptions.
Adding "2024" to the string will return only posts you made this year.
SYNTAX NOTE: there is no space between :site:" and "discussions."
As other end users we do not know the reason for Apple's decision.