  There is considerable evidence that the superconducting state of
Sr$_2$RuO$_4$ breaks time reversal symmetry. In the experiments showing time
reversal symmetry breaking its onset temperature, $T_\text{TRSB}$, is generally
found to match the critical temperature, $T_\text{c}$, within resolution. In
combination with evidence for even parity, this result has led to consideration
of a $d_{xz} \pm id_{yz}$ order parameter. The degeneracy of the two components
of this order parameter is protected by symmetry, yielding $T_\text{TRSB} =
T_\text{c}$, but it has a hard-to-explain horizontal line node at $k_z=0$.
Therefore, $s \pm id$ and $d \pm ig$ order parameters are also under
consideration. These avoid the horizontal line node, but require tuning to
obtain $T_\text{TRSB} \approx T_\text{c}$. To obtain evidence distinguishing
these two possible scenarios (of symmetry-protected versus accidental
degeneracy), we employ zero-field muon spin rotation/relaxation to study pure
Sr$_2$RuO$_4$ under hydrostatic pressure, and Sr$_{1.98}$La$_{0.02}$RuO$_4$ at
zero pressure. Both hydrostatic pressure and La substitution alter $T_\text{c}$
without lifting the tetragonal lattice symmetry, so if the degeneracy is
symmetry-protected $T_\text{TRSB}$ should track changes in $T_\text{c}$, while
if it is accidental, these transition temperatures should generally separate.
We observe $T_\text{TRSB}$ to track $T_\text{c}$, supporting the hypothesis of
$d_{xz} \pm id_{yz}$ order.
