I know nothing of why postgres don't want to implement them, but coming from the MS SQL Server side of things I can tell you they are quite an effective foot-gu

I know nothing of why postgres don't want to implement them, but coming from the MS SQL Server side of things I can tell you they are quite an effective foot-gun in a number of circumstances. More often, in fact, than they are a benefit. Often people reach for index hints when they should be fixing issues in the query or the available indexes, swapping a scan for many seeks which on small data does give a benefit but once code goes into production and the data grows performance falls through the floor. Also if they are named in the hint then you open up a new family of errors if the index gets altered later (though you could perhaps implement the hint as “use an index on columns x & y” rather than “use this specific named index” to get around this). There are circumstances where they are a genuinely useful tool, of course, but not nearly as many as people seem to think. Beyond that I assume implementation will have difficult points. If the hints are taken as instructions it may require significant changes to the rest of the generated query plan, if they are sometimes ignored then people will complain bitterly…