A little more information for anyone from Google: yes, this is still a thing in 1.16, and since it's been a thing since - I believe - 1.6, it likely won't be changed. It, as much as it pains me to say, really is that simple. I think the excuse is that allowing every texture to be rendered transparently would cause lag, but prior to 1.6 I would make textures fully transparent and not that much lag was caused. Perhaps something was changed in the render engine since then, but I digress: The only way of getting semi-transparent blocks is to retexture currently semi-transparent blocks - and no, you can't change the block model of a non transparent block to use a semi-transparent texture, you have to replace the texture of the semi-transparent block in question. But with all of that typed, the only time that isn't a viable solution is in my extremely oddly specific situation where I run a survival server where the admins use command blocks and plugins to do quests, and we want to recolor a certain block without changing it's texture directly so we can accommodate for different texture packs. Back in the day, the only other viable use for this was barriers in adventure maps, and those are now vanilla features of the game. So, understandably, it's of very low priority to change.
EDIT: I just realized this MIGHT be useful for resource packs as well, but for the rare instance that you actually need to do that, there are now free tools for creating block models (I feel like an old man having typed that. Gone, thankfully, are the days of that weird $20 program that did it for you...).
I know that you can have transparent item textures. You could try something with an item model, invisible armor stands, and barrier blocks to create a sort of custom block if you don't want to replace glass or ice. I know you said no mods, but with optifine, you don't have to replace any items either; you can create a name specific item that the armor stand could hold. All that could create a lot of lag in large amounts though.
Blockbench would be a great tool if you don't know how to make models, and Xisumavoid's Armor Statue datapack is good for posing them correctly. To place the barrier blocks, you would have to use the /setblock command.