Old and gold?

[chainazo]

Hello, I don't know anything about SEO, but I have a question that maybe you can know what happened here:
Until 2 years ago my website was made in html 4.01/css (and nothing else).
I ranked 1 for my own brand (usual) but I had 5 sub-items below that position (History, Products, etc.). Great because those 6 results took up a lot of serp.

BUT since it was unnavigable for mobile phones, two years ago, I changed it for a WordPress site with Gutenberg...result: I lost all the sub-items, only the first position remained.
A year ago they redid the site with Elementor looking for more "friendliness" for Google, but nothing changed, no sub-item was added.

Could it be that Google prefers and treats an old site but with pure code better than an "updated" site?. Thanks in advance.

[aristotle]

Are those other five pages still indexed? Or did they completely disappear from the search results?

If you set up the site in Google Search Console, and submit a site map, it might tell you if those pages are still indexed, and if they aren't, it might provide a reason.

Also, when the code was changed, was any of the content, and/or internal linking, also changed?

[chainazo]

Are those other five pages still indexed? Or did they completely disappear from the search results?

Yes, the links are still there and GSC shows it well.

Also, when the code was changed, was any of the content, and/or internal linking, also changed?

No, I kept everything the same in hopes of keeping what Google "had liked"

[lucy24]

BUT since it was unnavigable for mobile phones

Tangentially: Look into what makes a site responsive. It is absolutely possible to have a phone-usable site using only html and css.

[londrum]

I did a complete redesign once and google seemed to re-evaluate it from scratch. if you do a major code and design overhaul it's almost like they consider it a brand-new site

[not2easy]

I'll secoond what lucy24 said and add that it is far less maintenance dependent than WP. For a small site I would not bother with the overhead of WP.

Could it be that Google prefers and treats an old site but with pure code better than an "updated" site?

Not if it is not mobile friendly.