My reading is that OP was disappointed that they weren't leveled as senior, yet, they left the company because they were working on the tech they want to be an

My reading is that OP was disappointed that they weren't leveled as senior, yet, they left the company because they were working on the tech they want to be an expert of. It strikes me as a misunderstanding on how career evolution works in these companies (and, I believe, most tech companies). A senior eng will focus less on the pure technical aspects (language, framework, etc), but rather will have the soft skills allowing them to carry larger projects/impact. For instance, with experience, a senior can identify design mistakes early, can weight the value of different solutions based on aspects like time/eng-cost, tech debt, complexity, risk. In a company of the size of Shopify, shifting away from "Rails ERB templates" to a more modern solution definitely require these skills (and also probably way more than 6 months of tenure). They could have proposed this project, tried to get some buy-in and learn new "frontend skills".