I am going to guess measuring PSI today is a more reliable and accurate measure. Even if today’s loads have been watered down by decades of lawyers.
I seriously doubt lawyers have anything to do with it, unless, perhaps, they advise their engineer clients to rely upon the most current and accurate measurement methods.
NOT LEGAL ADVICE: Because of the inherent risk of reloading (as all the manuals prominently state), the risk exposure to a publisher of information is quite low. If the data is tested appropriately and conforms to the stated specifications, the risk exposure is
really low. It would be
very, very difficult to successfully maintain a suit under most states' laws against a publisher who had data that was backed with state-of-the-art testing. The causation issues alone (e.g., eliminating an overcharge by the handloader) would make it just about impossible. I think it is simply implausible that the in-house lawyers or outside counsel of powder companies or other publishers are telling them to back off any arbitrary percentage from what the data and the engineers' judgment supports.
Everyone likes to blame lawyers for all kinds of things, and sometimes they are right - I do not think changes in load data is something that lawyers have had any material effect on. At all. Testing methods have improved, and can now detect phenomena that the older test methods missed. Numerous industry folks have stated that.
Question: "Oh, but how can [old load] have been unsafe, I fired hundreds of those things and never blew up my gun!?"
Answer: Because properly-designed guns have a safety margin built into them, just as other engineered products do. If you build an elevator to handle 4000lb loads, only a reckless moron would design it so that the cable snaps at 400
1lbs. There's a safety margin built in.
Similarly, a gun intended to fire a 35k PSI round should not blow up just because it gets to 36k PSI. Indeed, firearms in many countries are subjected to "proof load" testing, where ammunition that is a specified amount
over pressure limits is fired in the gun to make sure it doesn't blow up. But that doesn't mean that firing proof loads continuously won't blow up the gun or otherwise damage it (or the shooter). You generally want to keep the safety margin as that - a safety margin.
There are some old published loads that were tested with old methods that live in the safety margin. Which leaves an unknown-but-reduced level of margin for anything else, such as a slightly hotter batch of powder or primer, or a bullet that gets set back a touch or jams the rifling a touch, or a lead-fouled throat, or a .1gr overcharge, or....