TLDR: this is likely a typo1.
AND PaidDate <=+ '2021-10-31' is parsed as
AND PaidDate <= (+'2021-10-31'), and the unary plus doesn't change anything so...
I'm speculating, but this probably has its roots here (emphasis mine):
Although a unary plus can appear before any numeric expression, it performs no operation on the value returned from the expression
And then they must have thought, the unary plus doesn't make sense with any non-numeric data type anyway, so why don't we just turn it into a no-op altogether, instead of coming up with data type-specific error handling.
And here you go, unary-plus whatever you want.
SELECT ++++6++++++++++++++++++++++++++++5 rightly returns 11, since it's just
SELECT (++++6)+(+++++++++++++++++++++++++++5), same as
1 - Notice that "=" and "+" are on the same key.