I've played this game over many years and operating systems until Win XP. So I bought it from here for Win 10. Either colony pods or sea pods disappear from the menu late in the game and are not even listed as obsolete under unit workshop. I have ran verify system files every time this happens and it has yet to find a problem. Same thing has happened to other unit types but don't recall which ones. I've played through three or games before this started happening. I rember now sea formers were the first unit type to disappear. T claify previously created units do not disappear just can not build new units.

Thanks to those of you that replied. While I have read some ideas that I haven't thought of in unit creation the rest have been questions that I already gave answers to in my original post. This is a software bug but I thought what the hell I'll post my issue anyway in the hope that someone else has run into this bug and had a solution. A long shot I know.

Disappear late in the game eh?

In the later game i often end up with allot of coastal cities going landlocked due to all the terrain raising/lowering going on.

If you enter the design workshop from a coastal city that is now landlocked, no ocean units will be available. (They wont even be visible in the design workshop)

Edit: sometimes it is hard to spot a coastal city got landlocked due to the city is visually quite big in its square.

Can you still design them in the workshop at coastal cities or are they just gone from everywhere?
(By which I mean, can you go to the design workshop and put a colony pod on a foil etc...)

The game will only allow a limited number of active designs in the design workshop; I think it's 64 maximum. The game might be automatically deleting old designs to make room for new ones. Check your design workshop to see if you have any empty spaces available. If not, delete some old designs and then design a new colony pod.

You might even want to make an upgraded one. Try a colony pod on a rover to get it to it's destination quicker. I've even made needle jet colony pods. Those are great for zooming over water and hostile terrain to start a new colony quickly! I also always design my own formers on a rover chassis and keep upgrading them. Try them. Faster formers are very helpful.

That has not been a problem

I have noticed Tthe number of design slots actually decrease late in the game. Osolete units can not be upgraded. I've tried designing new units as you suggested. The game will delete all units previously created using say the Air foil chasis with a colony pod. I might try that earlier in the game well before unit types disappear. I've also noticed that many changes were made to the original game. Volcano's and earthquacks were fairly rare in the original game. Gone everywhere. As I stated they are not even listed in the design workshop even under obsolee units.

So what part has disappeared exactly?
Is it the foil chassis or the colony pod or both?

The individual compents are still there. The Units themselves are gone in the workshop. Can't create a new unit without a base model as they are considered by the game as an upgrade. The units are completely gone from the workshop. As I stated they are not even listed as obsolete units. Basic unit types listed as obsolete can not be upgraded without lhe already built units in play in the game being deleted. There is no point in re manufacturing all those units at all let alone that late in the game.