I was originally about to answer the counterevidence... But wow, you are actually correct! I don't have a copy for ISO 8601-1:2019, but 8601-1:2016 [1] has the following note in the section 4.3.2 Complete representations:

> By mutual agreement of the partners in information interchange, the character [T] may be omitted in applications where there is no risk of confusing a date and time of day representation with others defined in this International Standard.

I thought this allows a replacement of [T] with a space, but it doesn't state so! It just means that the time designator can be omitted, e.g. 2023-09-0117:12:30 instead of 2023-09-01T17:12:30. It does look very strange (especially when used with extended formats) and I'm not sure if it was actually intended or not as there is no accompanying example, but nevertheless the 2016 spec doesn't allow a space. Oops.

EDIT: It indeed seems that ISO 8601-1:2019 no longer has this note. I now agree that this is a bad move. https://stackoverflow.com/a/9532375

[1] Specifically the Draft International Standard that was once available from loc.gov.