OmniGroup's policy is to charge for major upgrades (with a lower update fee charged to all prior owners), so version 4.0 was pay for, 5.0 was pay for and 6.0 will be pay for.
Version 4 was released pretty much the same time as OS X 10.0 (which is, what, over 6 years ago now?). Version 5 was first released as a beta in Feb 2004 and after several betas, went final in Aug 2004.
Since then there have been several 5.x and 5.x.x upgrades (all free).
You can see the timeline in the
release notes.
I've been a paid user of OmniWeb since version 4.0.6 (IIRC) which was released in 2001 or early 2002. I paid the Education discount price of $20* for it at the time (subsequently, OmniGroup has lowered the price they charge for OmniWeb to the $15 you will pay - it used to be $30). With 2 major upgrades at $10 a shot, I have paid a total of $40 for using OmniWeb for over 5 years. Not too onerous for the software, after the OS itself, that I use the most on my system (i.e. most days, and certainly every time I use my computer).
* all figures rounded up to nearest whole dollar.