I don’t think so. At least, not according to the Wikipedia page on stirrups. But even if they did, that doesn’t seem relevant to me, since they were used in Asia already.
“Technologies don’t require prerequisites”
For eco bonuses techs this is perfectly fine you have to skip Eco upgrades in order to get them for free next age great on open maps where skipping Eco upgrades means getting more early military. The amount of resources and time you can save on tech switches can be insane, especially on closed maps where you can easily reach imperial age without military and save 1470F 520G on BS upgrades alone. This is busted I would nerf it to “techs from previous ages are 50% cheaper and 100% faster.”
Honestly, this bonus seems out of place and doesn’t synergies with the rest of your civ bonuses I feel like you put it in because you felt like it was cool (which it is). But it would better synergies with a counter unit civ where your ability to make tech switches faster and cheaper allows you to rapidly respond to whatever opponent decides to make. Or a civ with an open tech tree which would allow them to access all of their options.
“Farms built around Mills last 20% longer”
This has much more synergy With a civ that specialize in Calvary it however it seems underwhelming when compared to other farm bonuses. I would buff significantly to farms around mills regenerate 5f/m, provide +1/1 per age, and healing to villagers. This would need to coincide with nerf to or even the removal of the prerequisite technology bonus.
“Knights benefit double from Husbandry”
sounds balanced considering they lack bloodline. keep in mind lacking bloodlines is less impactful with paladins with more base health.
Unique Unit 1: Haadlingen: The idea of a unit that buffs nearby villagers is interesting but I think this just makesemphasized text hussar raids unviable also I don’t think it is viable as a main battle unit.
Unique Unit 2: Soldnerheere
Interesting UU Idea but I’m not sure how you are supposed to use them. One of the main advantages of Hussar is their lack of gold cost in exchange for this the Soldnerheere gets +5hp and 35 hp/m. HP regen is important on Gold intensive units but not very much for trash units whose tactics rely on their disposability. Hussars are often used for sniping siege and raiding eco both have very low survival rates
which makes the Soldnerheere a worse option. Instead, you need to use more passively for hit-and-runs or small skirmishes. However, I would argue Hussars are perfectly viable in this role yes they have a higher turnover rate but replacing them is not as big of a issue since they are trash units. I would give them some sort of buff to help solidify their role maybe give them same speed as hussars to help them pick fights, add charge damage to help in short skirmishes or maybe 1ma armor to solidify as being better against melee units. Also make sure all light cav bonuses also apply to them
Friesian Husbandry: Light Cavalry +40 HP
This can be really powerful in Imperial age but especially in team games you are nerfed for not being able to research bloodlines for your scout cavalry.
Redjeven: Villagers and Hussars can be produced from Pastures and Mills
I don’t see it being as useful as you think by the time you would research this tech you are already fully boomed in a 1v1 in most situations where you need to reboom beyond the capacity of your town centers are probably already dead. Still, the ability to make villagers from a cheaper none stone costing building can be game-changing, being able to make hussar is also useful. Unfortunately, the tech is prohibitively expensive. As for pasture themselves I think you overestimate the utility of herdable in imperial age they require micro to properly utilize and with Hand Cart and Crop rotations farms become the superior option you could fix this by increasing the shepherd collection rate each age.
Overall 8/10 I still would not like to see this civ added into the game as is but every idea Bonus, UT, and UU was original and interesting. Normally I would reject European Cavalry civs on principle they have been done to death but you did a good job differentiating them from every other civ.
A few things i know the lowlands are known for include the goedendag uses by these people and flemish alike. The Beguine monastic woman for a unique priest option and the holk for a possible naval unit.