Is Sri Lanka a visa-free country for entry into Thailand?

No, you’ll still need to apply for visa before you come, (but you might be included in the possible coming new visa exemption scheme which still is a bit out in the blue).

Currently someone with a Sri Lanka passport can ONLY travel to Thailand if they apply for and receive a visa first. Also people from Sri Lanka cannot apply for the 30 day tourist visa extension, so if they get a tourist visa to Thailand they are limited to only 60 days before they must leave.

The most recent proposal to expand the number of countries which can enter Thailand without a visa (visa exempt) includes Sri Lanka, but as this is only a proposal, no one knows if or when it will come into effect.