There are two different concepts at work here:
1. R&D credits (IRC 41 https://www.law.cornell.edu/uscode/text/26/41). Companies can choose whether or not to pursue R&D credits. This is what Gusto was spamming you about.
2. R&E expenses (IRC 174 https://www.law.cornell.edu/uscode/text/26/174) which as of 2022 can no longer be fully deducted, but must be amortized over 5 or 15 years. IRC 174 (c)(3) explicitly states "any amount paid or incurred in connection with the development of any software shall be treated as a research or experimental expenditure." This applies whether or not the company was treating software development as R&D under IRC 41.
For more details, see https://www.striketax.com/journal/tcja-and-the-resulting-tax...
Note this would also only affect profitable companies (i.e., not most VC-funded startups), since there's nothing to deduct if you didn't make enough profit to owe tax in the first place (modulo some change in definition of "profit" based on how software development must be categorized - but still, this would only affect companies with fairly significant revenue; it's not like hiring a software developer suddenly costs 120% more than it did last year.)
They should have known they were taking a risk by adopting that strategy. At our company, we got a bunch of spam emails offering to help us file for R&D credits - we just ignored them and continued to pay normal payroll tax.
Searching my inbox for "R&D," it seems that Gusto was the most prolific spammer in this regard - they sent dozens of emails enticing us to save tens of thousands of dollars by talking to their R&D tax specialists. They even included case studies naming specific companies and how much they "saved." In retrospect, that looks like a big oof.
[0] https://www.aprio.com/its-official-software-development-incl...