A few things...
First, if you're using an indexer cluster, plan to just use master-apps on the cluster master (cm) to push config to your indexers. If that same server is also configured as a deployment server, use that to manage other components of infrastructure. But note that it is recommended (as you scale at least) that the cm is just a cm and nothing else.
Second, we typically create apps/add-ons for our configuration. For example, if I work for the ACME corporation and we onboard Application ABC, then we may create an add-on called: TA-acme_abc. In that app would we would have a local folder where we put our configuration. And we would have metadata folder where we would create a simple local.meta file. And then we would put that TA in the master-apps folder and push it out. If you also have a search-time settings in there, you could also put it in deployment-apps and push it to your search head.
Third, for your actual conf files. I'm not sure if that's a copy/paste or something you just typed up when created the post. But case matters. All of the settings on the left side of the equal sign should be in all caps. Also, I'd suggest adding the description part to your transforms settings just to be sure it's unique. Also, if you specify source:: in the stanza name, it expects the source not the sourcetype. If you want to use sourcetype, then don't specify a prefix...it's the default behavior for a stanza.
So maybe like this:
props.conf
[splunkSourcetype]
TRANSFORMS-mask_sensitive_data = mask_string
transforms.conf
[mask_string]
DEST_KEY = _raw
REGEX = regex
FORMAT = replacement
Also if possible, it's helpful to have a test/dev environment....even if it's just a standalone splunk instance. That way, you can create your add-ons there, manually upload sample data and verify that it works the way you want before pushing it your infrastructure. We don't want to keep pushing to production while trying to get something working.
And one final side-note, you could also use SEDCMD in props at parse time to mask data....if you wanted.
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