I have been in the electrical trade for 20 years. This is a common practice, but I advise against it. It is a code violation because it gives users a false sense of security- thinking there is a ground. If/when you sell the house it will be falsely representing the condition of the electrical system. (Although most buyers hire an inspector who would probably catch it.) I admit adapters seem sketchy, and if the fork terminal is not under the cover plate screw or if the junction box is not grounded (probably isn’t) then the adapter also could give a false sense of security—but more people would be aware of the sketchiness which is a good thing. There are safe and code compliant ways to add a three prong receptacle short of rewiring. If the box is metal and well grounded with metal conduit or armored cable you are permitted to ground the receptacle to the box. Also you are permitted to install a three prong GFI and use it to protect 3 prong receptacles downstream, provided each one is labeled “GFCI protected: No Equipment Ground.” (These labels come with the GFI.) Also, short of rewiring the entire house, you (or your electrician) can strategically add new wiring to critical locations. (Washer, computer, garage, sump pump etc.) I don’t want to seem like I am coming down hard on you for suggesting this solution. Just giving you my opinion.
My house (1962) is a bit of a grab bag for power , seems original was two prongers yet the old Romax actually has a ground wire ... have been able to add some threes and one GFI and feel safe(r). If you have fuses it's probably much older but check the wire anyways.
Our home built in 1959 had two prongers, but each metal box had 14-2 or 12-2 Romex running to it with the ground wire twisted around and under the head of one of the Romex clamp screws. Pigtailed these short ground wires with a longer ground wire, recapturing the box and the ground lugs on the three prong receptacle upgrades. The inspector’s one requirement was that he had me pigtail all the devices rather than running them in series, thus one bad receptacle wouldn’t affect the others.