[R-meta] A potential addition to metafor random-effect structures

Hi All,

From time to time, I encounter situations where the number of levels

for a categorical variable and/or a combination of such variables are
large enough in each study (i.e., creating high-dimensional joint
effect distributions) that I need to forgo adopting a "UN" structure
associated with those levels in favor of a more restricted structure
(e.g., "HCS").

Depending on my research goals, however, this strategy may be suboptimal.

Recently, I noticed that the glmmTMB package has added a new
random-effects structure for these cases called the "reduced rank"
structure possible by imposing some restrictions on the matrix of
random-effects to ensure the relevant parameters' identifiability.

I'm not sure how much and/or what kind of assessment(s) of this new
structure currently exists in the methodological literature. But on
its surface, it seems that this might potentially offer some solution
to the problem described above.

Will be glad to hear your thoughts/comments on the potential of this
new structure for multivariate-multilevel meta-regression models
perhaps implemented in rma.mv().

Kind regards,
Reza