Defender Sub-Systems not being researchable?

I have had to play Mission 5 a few times so that I can leave with a slightly better fleet than I started. However, in a couple of the playthroughs, the mission point that allows this item to be researched doesn’t pop.

I don’t remember what this technology opens up. Also, if I don’t get this on this mission, will it be available in a subsequent mission, or is it gone and I need to replay this mission?

Usually that tech becomes available when you spot enemy defenders. The idea is that your fleet likes what they see and decides to reverse-engineer them. If you somehow beat the level without running into any defenders you might not get the technology.

However, usually you get tech like this at the beginning of the next mission. I know you get ions and drone tech the next level if you don’t buy it from the Bentusi.

You don’t really need it anyways - it is better to just build frigates and then DDs/HCs. The AI will never use enough strike craft to endanger a DD (let alone a HC) and you can just carry a few drone frigates to screen out anything. A drone frigate is basically ~12 defenders on a frigate hull with 18000 HP.