My text editor complains when I type 'startpoint' and does not complain when I type 'endpoint'. Why does this difference exist and what should I use for each?
Your text editor has a dictionary that it consults in order to decide whether to complain about a word you type. This dictionary has an entry for endpoint but not for startpoint.
Why is that? It's hard to be sure without knowing which text editor you are using and where its dictionary comes from, but it is likely that one of the techniques used to compile the dictionary was to consider the most frequently used words in some corpus. And endpoint is a more frequently used word than startpoint, so it is more likely to be included in such a dictionary.
The OED notes that startpoint is rare:
start-point n. rare = starting-point n.
1876 J. Ruskin Fors Clavigera VI. lxii. 59, I find myself..without any start-point for attempt to understand them.
And here's a chart from Google Ngram Viewer showing their relative frequencies:
There is a mismatch in the antonyms, because we pair starting-point with end-point.
One of the main reasons, is they came in at slightly different times, for different cases, and don't always have a pair.
We generally define the starting-point in terms of an action. Something must be done to trigger whatever is started.
We generally define the endpoint in terms of an effect; whatever was started at the starting-point will often come to its endpoint itself.
A 19th Century man of ideas (they are both from that time) would generally cause starting-points, but only observe endpoints.
They also come originally from different fields, starting-point being used to describe several endeavours (from philosophy to going on a walk) while end-point was originally used to describe observed chemical processes at which there was a clearly observed change at some point.
As such, they're really only near-antonyms. But when by extension one wanted to talk of the opposite of an end-point, one used starting-point, and when one wanted to talk of the opposite of a starting-point, one used end-point.
End-point is also more likely to be used closed as endpoint than starting-point is as startingpoint, primarily because closing a compound where the first part is an -ing gerund is unusual.
Indeed, of the few cases of startingpoint found in this ngram search, most seem to be OCR errors either missing a space in the open form ("starting point") or mus-identifying a hyphen at the end of a line as a soft rather than hard hyphen: