Are there any restrictions for finding Rare/Legendary loot?

In Fallout Shelter, are there any restrictions for finding Rare/Legendary loot?

I am asking specifically about the recipes; but if there are restrictions for finding Rare/Legendary loot in general (like in the Wasteland or in Lunchboxes) this is relevant to my question.

For example, if I have crafting rooms, but I have not upgraded them yet (either because I don't have the dwellers or the caps), can I still find recipes, or will I only be able to find them once I have upgraded the room?

You can find recipes even if you dont have the rooms at all, also you find them when your room is not upgraded, but you only see them after upgrading the room.
I had collected a lot of legendary weapon recipes before upgrading my weapons-room, after upgrading they all became visible there.
You can only drop every recipe on time.
I had sometimes dwellers which found 3-4 but more often dwellers wich find none or one, my personally expirience is, a max-special level 50 dweller exploring wastelands till 100 items finds an average of 1 legendary recipes during his exploration.
But you can also find legendary recipes with a level 1 dweller in a new vault, but only with a low chance.

As far as i know there are no restrictions for the recipes, I find them often when dwellers explore wastelands and had collected all available armor recipes in 2-3 weeks and weapon recipes after 5-6 weeks.

About items it is more difficult. There are some older information about this:

There is a statement that you have a chance 10% cance at the national guard depot when exploring 60h the wasteland https://www.reddit.com/r/foshelter/comments/3bn3s6/legendary_success_rate_1_in_10_from_national/

Also there is a statement you can get them as random drop but only a very small chance depending on the time you spent in the wasteland: https://www.reddit.com/r/foshelter/comments/3bex4b/speculation_legendary_loot_chance_staying_longer/

According to this post the chance getting a legendary item from a lunchbox is 5% https://www.reddit.com/r/foshelter/comments/3a1o4t/analysis_of_40_opened_lunchboxes_statistics_and/
I often had good legendary weapons or armor in lunchboxes at the start of a new vault but also in old 200 dweller vaults, there is no restriction to this chance depending on other factors.

But I am not sure if these statements still are valid after the last updates 1.4 and 1.5 (1.6 comes in July). It seems that after introduction of recipes and armor/weapon rooms you cant drop them in the wastelands anymore at all but I found no final statement that confirms or reject this: https://www.reddit.com/r/foshelter/comments/49u4n3/drop_rate_changes_with_14/

Also I dont know if introducing the scrap items did change the lunchbox chances, my personal experiences says no but I can not back this up with sources.

I personally, after 3 months of playtime when update 1.4 was released, with several vaults and always 25 max-special level 50 dwellers in the wasteland for 3 days or longer, never dropped a legendary item in the wasteland, got them all from lunchboxes or builded them myself, so I can only confirm, if there is a chance of getting legendary items (except recipes) at wasteland at the moment, then only after long exploring time and it is very very low.