Yes, phosphorus is essential to plants. It's important for humans, too (in moderation), but it's one of the three major components of fertilizer for plants (the P in N-P-K). Phosphorus helps with flowering, roots, photosynthesis, cold-tolerance, plant maturity, leaf size, etc.
The major concerns I've heard about phosphorus are with supplemental phosphorus (as opposed to phosphorus naturally in the ground), and are environmental, and ethical, but there are some health concerns, too. It all depends on what kind of phosphorus you're talking about. It's pretty difficult to find supplemental phosphorus that everyone can agree is healthy, fine for the environment, etc., including when it comes to organic forms of supplemental phosphorus.
Sources of phosphorus (including organic and non-organic forms):
- Bat guano: this sounds like it works great, and is organic, but people worry the harvesting of it is harmful to the existence of the bats.
- Super triple phosphate (it has a lot of names): it may be contaminated with fluoride, which may cause issues with plants, and potentially with people, too; I believe it's water soluble (not sure), and may leach into the water table if used in high amounts; there are other ethical issues with this and phosphoric acid production, but I don't want to delve into that, since I don't have all the facts and what is and isn't fact with me (nor does probably anyone). However, if you look up the phosphorus industry you may find some stuff. stormy shared a video in the garden shed about it.
- Bone meal: The animals may or may not have been given stuff you don't want in your garden.
- Wood ash: Although I personally don't think it's a significant amount, and I love using wood ash, they do contain some heavy metals. Wood ash has about twice as much potassium as phosphorus, however, and a lot of calcium by comparison (so it can raise soil pH).
- Monopotassium phosphate (if phosphoric acid is used to make it, then the fluoride issue may exist here, too, but I don't know if that's an issue; it's water soluble—so it may leach into the water table if used in large amounts; if you've got clay soil, I imagine the leaching isn't as huge of an issue; I've used this with good results, although I'm still wondering how much fluoride is in it, if any; it has a lot of potassium in it, too)
- Monoammonium phosphate (ditto, except it has a little nitrogen in it instead of any potassium)
- Compost: People don't generally go on about how bad compost is, but it's only as safe as what your compost is made out of. If you put organic waste in it covered in pesticides and herbicides, you might have the same issues as with the bone meal.
- Worm castings: these aren't really a phosphorus source, but they may contain microbes to help make existing phosphorus in the soil more available. I could be wrong about that (since maybe I'm not remembering right): maybe it was mycorrhizae that did that, but worm castings may have mycorrhizae in them for all I know.
Water pollution is a concern with water soluble phosphorus; so, that may be what people are concerned about with the toilet cleaner.
Anyway, my source is just research all over the Internet, which I've done the last few years. It's not all indisputable, but this is what I recall.
In reading the claims against forms of phosphorus in this answer, be careful not to assume that I take any particular stance on any particular thing I was talking about (with the exception of wood ash, which I think is great stuff, despite claims against it).