legal thickness limit for sealant

There is no set limit on thickness. The rule states that the blade is the racket without the rubber. 85% of the blade must be of natural wood and can be in any shape, size and weight. Since the sealant is apply directly onto the blade, it is consider part of the blade and thus subjected to the 85% rule.

Back in the day, Wang Liqin fans here used to claim that he literally soaked his stiga off in shellac.

Baal wrote: ITTF Technical Leaflet T4 says there is in fact a legal limit as to how much sealant you can use on the surface.  http://www.ittf.com/stories/pictures/T4_Racket_Coverings_forBoD42014.pdf A. 2. Blade A very thin layer of lacquer is permitted on the blade, only for the purpose of anchoring wood fibres, thereby facilitating replacement of the racket covering. Anything more than this will be deemed to constitute a layer of plastic, and will not be permitted. This layer may be no more than 0.1 mm thick, and should not hide the wood from sight or touch. It is considered to be part of the blade, rather than part of the thickness of the racket covering.

Correct, that's the recently changed rule...

That's mainly due to the methods of rubber thickness measurement at events.
Many Pro's started to apply a partial, rather thick layer of sealant on the "wings", but not to the rest of  blade surface - this helps to stay in the "safe" area below 4.05 mm - as the feeler tip of the dial gauge measures on the wing surface. A 1/10 of varnish in that place can help quite a lot - some more even more....

Jay is correct, it is not anything very new.  I don't know if it's a rule, but if you are at a tournament, they can disallow a blade if they see it is in violation of the directives in those leaflets.  Technical leaflets are also the basis for how they will put their stamp or approval on balls, rubbers, nets, tables, etc.  They do change them from time to time.