With $x=2$ and $b \ge 1$ the claim is **true**.

Proof: Since $G$ has no degree-$1$ vertices, it is not a forest, so it contains at least one cycle. The cycle is your $2$-regular subgraph.

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With $x=3$ and $b=4/3$ the claim is **false**. The following $(3,4)$-biregular graph has no $3$-regular subgraph.

[![(3,4)-biregular graph with no 3-regular subgraph][1]][1]

This example is Figure 2 in Asratian et al. (2009). They note that it has no *full* $3$-regular subgraph (*full* meaning "one that covers all of the $4$-degree vertices"), but we can in fact find that it has no $3$-regular subgraph at all. It suffices to consider all $2^8-1=255$ nonempty subsets of the $3$-degree vertices (lower layer); in each case, take the subgraph of those vertices and all their neighbors; and observe that in all cases the resulting subgraph is noncubic. (This was, of course, a straightforward computational way; surely there are finer methods.)

To provide some context (and to address the question "which mathematical fields are likely to be helpful"): It seems that regular subgraphs of biregular graphs are relevant in *interval coloring*, which is edge-coloring with certain constraints. Most results that I quickly found are in the direction "*if* there is a regular subgraph with so-and-so properties, *then* you have an interval coloring".

<cite authors="Asratian, Armen S.; Casselgren, Carl Johan; Vandenbussche, Jennifer; West, Douglas B.">_Asratian, Armen S.; Casselgren, Carl Johan; Vandenbussche, Jennifer; West, Douglas B._, [**Proper path-factors and interval edge-coloring of (3,4)-biregular bigraphs**](http://dx.doi.org/10.1002/jgt.20370), J. Graph Theory 61, No. 2, 88-97 (2009). [ZBL1198.05037](https://zbmath.org/?q=an:1198.05037).</cite>

  [1]: https://i.sstatic.net/hSA0V.png