I've used it a lot for steel joist web reinforcing. Never had any issues. Older (6th edition?) AISC has properties all the way down to L1x1x1/8 I think.
Not sure of the reason why they removed it. I would be curious if someone has the answer. I am always careful when designing with that angle and a bolted connection. Any hardware bigger than 5/8" dia. begins to have fit-up issues due to the conflict with the washer and angle leg. Also, you won't make the minimum edge distance requirements and will have to look at rationalizing the reduction of that provision. Obviously, a welded connection will not have this issue. I have designed many projects with it rather recently so IMO, yes, you can still use it.
It's been a couple years, but a colleague of mine had an available section list from a steel supplier. It had several small angles like that available that weren't listed in my steel manual. I suspect the reason they aren't in the steel manual is because they aren't generally used as a "structural section". Due to the reasons listed by STrctPono, there can be issues with connections.
A quick call to a steel supplier can confirm availability, but I suspect they are available.
You can use it if you can find it. The manual just shows the statistical "averages" of the common shapes available from AISC certified mills. If a shape is no longer in production, too few mills will roll it, they'll drop it from the book. Always a good idea to verify availability of sections not listed, as even some listed sections are only available if the fabricator/service center puts in a special order to the mill.
