"LaTeX3 Error: Command '\textsi' not yet defined!"

[David Pesetsky]

Before (June 2019, via MacOS Time Machine):
LyX 2.3.2
TexLive 1.35
MacOS Mojave (10.14.x)

Now
Lyx 2.3.4.2
TexLive 1.36
MacOS Catalina (10.15.3)

Minimal Working Example:
without embrac package:
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