Thanks for reporting this issue! We have discussed this as a team, and we consider it low priority for now. This is because: a) there are multiple ways that users may unwatch a page, either intentionally or unintentionally (such as clicking on the half star), rather than just the API, b) the use case of accidentally unwatching a page and then wanting to see former watch period is probably not very common, and c) if a user unwatches a page, they will be able to pick a new temporary watch period, even if they don't remember the exact former watch period. For these reasons, we may look at this issue later on, but it is not critical for the development of the MVP release. I'm keeping this ticket open, in case we have time and interest later on in looking into it. Thanks again!

In T254141#6313332, @ifried wrote:

Thanks for reporting this issue! We have discussed this as a team, and we consider it low priority for now. This is because: a) there are multiple ways that users may unwatch a page, either intentionally or unintentionally (such as clicking on the half star), rather than just the API, b) the use case of accidentally unwatching a page and then wanting to see former watch period is probably not very common, and c) if a user unwatches a page, they will be able to pick a new temporary watch period, even if they don't remember the exact former watch period. For these reasons, we may look at this issue later on, but it is not critical for the development of the MVP release. I'm keeping this ticket open, in case we have time and interest later on in looking into it. Thanks again!

Example use case is at T263665: Add handling for expiring watchlist items on Special:GlobalWatchlist, though since T263796: Watchlist query api should report time remaining would be required for that anyway, this might not be needed.

"if a user unwatches a page, they will be able to pick a new temporary watch period, even if they don't remember the exact former watch period" - this doesn't appear to be the case. Reported at T263664: Watchlist Expiry: watch/unwatch links should remove clock icon - when rewatching via watchlistunwatchlinks no expiry option is given