I think RNA memory hypothesis is not popular because there exist better, less controversial memory systems in the cell. For example gene regulatory network or bioelectric circuits in the cell (or, more likely a combination of them).
Biswas, S., Manicka, S., Hoel, E., and Levin, M., (2021), Gene Regulatory Networks Exhibit Several Kinds of Memory: Quantification of Memory in Biological and Random Transcriptional Networks, https://www.cell.com/iscience/fulltext/S2589-0042(21)00099-7
The bigger problem with most of these memory systems is eliciting correct behavior of amnesia in case of both electric and chemical shock, as well as permanence or lack of permanence of those amnesic inputs.
Biswas, S., Manicka, S., Hoel, E., and Levin, M., (2021), Gene Regulatory Networks Exhibit Several Kinds of Memory: Quantification of Memory in Biological and Random Transcriptional Networks, https://www.cell.com/iscience/fulltext/S2589-0042(21)00099-7