The answer is merely a matter of definition of "valence electrons".

Many texts define "valence electrons" in a way that explicitly includes $d$ electrons of unfilled (but not filled) $d$ subshells.

For example: Chemistry structure and dynamics by Spencer et al. at page 125:

We can define valence electrons as electrons on an atom that are not present in the previous rare gas, ignoring filled $d$ or $f$ subshells.

Many books published in the last 10 years use this definition.

That $d$ electrons may be valence electrons is also supported by the 18-Electron rule (at least to the extent that there is such a rule).

The answer is merely a matter of definition of "valence electrons".

Many texts define "valence electrons" in a way that explicitly includes d electrons of unfilled (but not filled) d subshells.

For example: Chemistry structure and dynamics by Spencer et al. at page 125:^[1]

We can define valence electrons as electrons on an atom that are not present in the previous rare gas, ignoring filled d or f subshells.

Many books published in the last 10 years use this definition.

That d electrons may be valence electrons is also supported by the 18-Electron rule (at least to the extent that there is such a rule).

[1] James N. Spencer, George M. Bodner, Lyman H. Rickard: Chemistry: Structure and Dynamics, 5th Edition. John Wiley & Sons: 2010. ISBN: 978-0-470-58711-9

The answer is merely a matter of definition of "valence electrons".

Many texts define "valence electrons" in a way that explicitly includes d electrons of unfilled (but not filled) d subshells.

For example: Chemistry structure and dynamics by Spencer et al. at page 125:

We can define valence electrons as electrons on an atom that are not present in the previous rare gas, ignoring filled d or f subshells.

Many books published in the last 10 years use this definition.

That d electrons may be valence electrons is also supported by the 18-Electron rule (at least to the extent that there is such a rule).

The answer is merely a matter of definition of "valence electrons".

Many texts define "valence electrons" in a way that explicitly includes $d$ electrons of unfilled (but not filled) $d$ subshells.

For example: Chemistry structure and dynamics by Spencer et al. at page 125:

We can define valence electrons as electrons on an atom that are not present in the previous rare gas, ignoring filled $d$ or $f$ subshells.

Many books published in the last 10 years use this definition.

That $d$ electrons may be valence electrons is also supported by the 18-Electron rule (at least to the extent that there is such a rule).