Is steam gift region-restriction still applicable?

Only +10%. You can gift to any cheaper region. This is why it is a bit funny to be Western/Central European on Steam, because apart from a very handful of exceptions, we can gift anything to anyone (as long as it does not carry a regional lock), but cannot receive almost any game from any other region.

The game needs to have an actual region-specific repo. Previously, entire Steam was region-separated; this is what they removd with the new system. but region-specific repositories are still in effect, so for example, you still cannot gift Metro 2033 Redux to Russia, because it has a CIS-only repo. But you can gift it now from US or EU to South America, which was not possible in the old system.

In my experience, you can gift it and won't get any warning at the checkout. The receiver will still receive it, but the warning will be displayed on his side, and when he tries to add it to his library he won't be able (if he is outside your region), his only option is to refuse it.

I'm from Argentina and here we still have the US store prices. In the past we had the region restriction that our gifts can be only activated on Latin America. Now if I gift a game to someone living in the US (the prices are the same) I can gift it, but it happens what I've described above. Now we are fucked up, cause most Latin America countries have their own currencies with better prices than we, so you can only gift to Latin America due to region lock, but you can't gift to most countries in latam cause price difference....

Correct. For example, Indonesians cannot gift almost anything to any country, even in SEA. Similarly, Russians cannot gift anything to any other CIS country, or Mexicans and Brazilians often cannot give games to other South American countries.

This is why SteamGifts has a country-based region system now, you need to be aware of which countries have their own currencies and lock Steam gift gifting based on that. In most cases, it means you pretty much need to gift to the same currency region, in your case the country you live in.

The one you linked can be gifted from the Philippines to anyone in Philippines, Russia/CIS, Turkey, Indonesia, ?alaysia, Mexico, and Brazil, and maybe to India and Thailand; but no other countries.
Nice game pick, by the way.

I would say you can. The difference isn't exactly a 10% always, and not sure how they calculate it. It's always some number between 10%-15% difference. Imo, you will be able to gift it to Singapore. But I can't guarantee that. Some times a price difference of 14% was gifted without problems, but another with 13% difference wasn't. So it's really strange how they choose this...