TYPO3 Core

TYPO3 version should probably be selected as 11, not possible, so I selected 10.
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In TYPO3 9.5 this still works if used in a Flexform for type select:

<settings.whatever>
  <TCEforms>
    <label>whatever</label>
      <config>
        <type>select</type>
          <items type="array">
             <numIndex index="0" type="array">
               <numIndex index="0">first option</numIndex>
               <numIndex index="1">1</numIndex>
             </numIndex>
             <!-- etc --> 
          </items>
          <size>15</size>
          <maxitems>100</maxitems>
          <renderMode>singlebox</renderMode>
       </config>
    </TCEforms>
</settings.whatever>

"Using the TCA field type `select` without specifying a valid `renderType` has been marked as deprecated. Additionally the usage of renderMode for select fields has been marked as deprecated." (since 7.6)

https://docs.typo3.org/c/typo3/cms-core/master/en-us/Changelog/7.6/Deprecation-69822-DeprecateSelectFieldTca.html