Yeah I'm getting loads of these errors too, seems to be for just about every extra I install.
To add to the list of known offenders
RefreshCache
Gridclasskey
Those two at least gave a message in the error log that it was them. As for the rest, it looks like this
[2019-01-08 14:52:26] (ERROR in modProcessor::run @ /home/hotlycom/ampli.buzz/core/model/modx/modprocessor.class.php : 177) Flat file processor support is deprecated since version 2.7.0.
[2019-01-08 14:52:30] (ERROR in xPDO::getService @ /home/hotlycom/ampli.buzz/core/xpdo/xpdo.class.php : 1234) modRestClient::__construct is deprecated since version 2.3.0. Use the modRest classes instead.
[2019-01-08 14:52:30] (ERROR in modRestCurlClient::__construct @ /home/hotlycom/ampli.buzz/core/model/modx/rest/modrestcurlclient.class.php : 26) modRestClient::__construct is deprecated since version 2.3.0. Use the modRest classes instead.
Only 10 times longer than that.
Anyway, turned off the log deprecated function as suggested.
it's in the System and Server Settings in case anyone's wondering where to find it, about third up from the bottom.