I'm reasonably sure that isn't the case. SRD is public domain.
The d20 SRD or the 5.1 SRD are copyrighted by Wizards of the Coast.
System Rules Document
Copyright 2000, Wizards of the Coast, Inc.; Authors Jonathan Tweet, Monte Cook, Skip Williams, based on original material by E. Gary Gygax and Dave Arneson.
System Reference Document 5.1
Copyright 2016, Wizards of the Coast, Inc.; Authors Mike Mearls, Jeremy Crawford, Chris Perkins, Rodney Thompson, Peter Lee, James Wyatt, Robert J. Schwalb, Bruce R. Cordell, Chris Sims, and Steve Townshend, based on original material by E.
Gary Gygax and Dave Arneson.
I'm reasonably sure that isn't the case. SRD is public domain. You can use, quote, and refer/hyperlink to it all you want, for any purpose. Even a straight-up competitor game system. You can take the SRD, rewrite every spell and item description, and publish that as your own work. Because it is your own work. TSR/WotC can't copyright the game mechanics, only text descriptions.
The only exception in the United State is Title 17, Section 102(b)
www.law.cornell.edu
(b) In no case does copyright protection for an original work of authorship extend to any idea,
procedure,
process,
system,
method of operation,
concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.
However, the expression of a procedure, system, etc. is protected by copyright. Hence rewriting game mechanics in your own words is likely OK.
But it is an open question whether a specific arrangement of methods and concepts is also an expression protected by copyright. Hit points are not protected, Armor Class is not protected, rolling 1d20 is not protected. But if your work has hit points, armor class, and 1d20 it may be considered a definitive work if the court decides that specific combination is a creative expression thus covered under copyright.
However there is a evidence that is not the case as the copyright office explicitly warns about this.
www.copyright.gov
The idea for a game is not protected by copyright. The name or title given to the game and the methods for playing it are not protected by copyright.
Some parts of a game may be subject to copyright if they contain a sufficient amount of literary or pictorial expression. For example, you may be able to register the text describing the rules of the game or the graphic art appearing on the gameboard or container.
Form TX can be used to register all copyrightable parts of the game, including any pictorial elements. When the copyrightable elements of the game consist predominantly of pictorial matter, Form VA should be used.
The part I highlighted in bold is what traditionally what makes publishers nervous in trying to work with D&D related material prior to the year 2000 and the OGL. For all its flaws the d20 SRD under the OGL 1.0a and later the SRD 5.0/5.1 provided certainty that Wizards was OK with the use of the material and won't try to argue every point as long you follow the license. That is until Hasbro/Wizards decided they would be better switching to the Chaotic Greed alignment.