Most people stop at 40

In general, the soft cap for most stats is 40, sometimes closer to 50, but anything beyond that is mostly pointless, especially with things like attunement, which gives nothing past 50, and endurance, which stops giving stamina (still gives carry weight) past 40

Soft is 40, hard is 50.

For STR/DEX the soft cap is 40, for INT/FTH the soft cap is 50 in relation to physical/magic weapon scaling. It has to do with how much Stat Scaling percentage you gain per level, and the breakpoints are different between the physical and magic scaling. The Weapon Scaling page has a couple of handy tables illustrating this. Spell damage scales with Magic Adjustment which is dependent on the Catalyst/Talisman. Generally, Catalysts and Talismans have drastically reduced increases in MagAdjust past 50 INT/FTH to where you are only getting a 1 point increase every few levels instead of 1-4 points per level. Other stats (such as Attunement/Endurance, etc.) also have varied soft caps as mentioned above.

40 normally, 50 if you want to use the heaviest strength weapons like the Demon's greataxe and havel's greatshield

Yes.

You do realise it said strength scaling not damage scaling right, meaning if you had 16 strength you can wield the zweihander with 2 hands instead of 1 handed, this does affect the damage however, just with the same stat grade scaling it had before so if you got a 50 bonus on a 200 damage weapon you'd have 250 total, two handing the weapon would give you 50% extra on that bonus making the total damage 275

Hey, if it works, it works.

not in the first dark souls game

Not at the base weapons, but some lpgraded weapons like Demon's Greataxe+14 did get S scaling