Is it allowable in Mainland China to use Traditional Characters?
some tidbits as i known, or heard:

calligraphy must be in traditional chinese, no matter in which scripts (cursive, regular, or seal)

so, signboards, billboards could be in traditional chinese, such as:

it used “報”, instead of “报”; anyway, it’s mr mao’s handwriting  no one dares to change it

classical / literary text should not be changed into simplified chinese, any reprint of “old books” (古籍) must be in traditional chinese.

there was an incident, quoted in the book “毛澤東晚年過眼詩文錄”: that in 1973, during cultural revolution, books complied for mr mao’s reading were printed in traditional chinese

another “incident” is, from 2003 to 2018, the book “全宋筆記”, 102 volumes, are also printed in traditional chinese.
back to your question:

people using Traditional characters in Mainland China just because they prefer it an no one cared. Is this true?

yes, most of the time, no one cares.

would I get weird looks or come off as odd/eccentric for using them in daily life such as to text a friend, colleague, etc. or write a document for someone

most likely, just tell them that you were/are learning chinese language in traditional script 
edited.

niche, or revelation

the logo of the some universities in mainland china:





these are in traditional chinese, so, the interpretation of such phenomenon would be . . . 
replying to a “disinformation” comment:

The company seal or personal name seal must go with simplified Chinese in mainland cn

even the national seal (國璽) of mainland china is in traditional chinese:

it’s “華”, not “华”; “國”, not “国”
it’s mentioned in another thread: Why are traditional and simplified different in such small manner?
the “Law of the People's Republic of China on the Standard Spoken and Written Chinese Language” (中华人民共和国国家通用语言文字法)

第十七条　本章有关规定中，有下列情形的，可以保留或使用繁体字、异体字：


　　(一)文物古迹；


　　(二)姓氏中的异体字；


　　(三)书法、篆刻等艺术作品； (art works, such as calligraphy and seal carving)


　　(四)题词和招牌的手书字；


　　(五)出版、教学、研究中需要使用的；


　　(六)经国务院有关部门批准的特殊情况。

seal must be in traditional chinese