I'm trying to determine if it is worth purchasing WiFi for my flight and am looking for experience with availability of United WiFi when exiting the continental United States. I will be traveling on UA1266 from Houston to San Jose, Costa Rica (IAH-SJO) on March 4, 2020, and then returning on UA1097 on March 8, 2020. According to my itinerary these flights will be served by a Boeing 737-800, and according to https://www.unitedprivatescreening.com the flights will have Thales WiFi on board. However, according to United, Thales WiFi is only available within the continental United States. Does this mean that the WiFi will shut off once the flight exits US airspace? If so, this would mean that the flight would only have WiFi available for a very short period of time, as the distance between Houston and the US border is very small, and I likely wouldn't bother purchasing the WiFi.
As you've said, United uses different WiFi systems on different aircraft, and the system (currently) used on 737-800 does indeed only provide internet access whilst (roughly) over the continental US.
However WiFi access is available for the entire flight.
Whilst many people conflate "wifi" and "internet" to be the same thing, they are very different - especially on United flights where WiFi is usable even without internet access to access things like streaming Movies/TV/etc and flight details. These features DO continue to function even when outside of the continental US.
Internet access, which is accessed via the same WiFi system, is as mentioned only available when (roughly) over the continental US.
You've got it right. United uses a few different WiFi providers depending on the aircraft. For 737-800 flights south of the US-Mexico border, there is no coverage, and the internet will stop working not long after departure from Houston. They should make an announcement to this effect at some point on board, but there's no point in purchasing WiFi for such a short period of time.