Prior to Japan’s script reform of 1900, there was more than one shape associated with each syllable of the hiragana syllabary. There is now only one shape associated with each syllable. The now-obsolete and nonstandard shapes are referred to as hentaigana (変体仮名), which simply means variant kana. Hentaigana are still in use today in Japan, but are limited to Japan’s family registry (戸籍 koseki in Japanese) and specialized uses, such as business signage and other decor that are specifically designed to convey a feeling of nostalgia or traditional charm.
In addition to the Wikipedia article that is linked from the previous paragraph, 『変体仮名のこれまでとこれから—情報交換のための標準化』 (The past, present, and future of hentaigana: Standardization for information processing) by TAKADA Tomokazu (高田智和) et al. and About the inclusion of standardized codepoints for Hentaigana by YADA Tsutomu (矢田勉) serve as excellent reading material.
Although there was an earlier proposal from early 2009 (see WG2 N3698 or L2/09-099) that didn’t seem to go anywhere, the first official proposal from Japan to encode hentaigana was submitted in 2015 as WG2 N4670 (please note that the link is to a ZIP file that will download if clicked; aka L2/15-193), and included 299 characters. The proposal was first discussed at UTC #144, and I was given the action item to relay UTC feedback to Japan. Subsequent revisions (L2/15-239, L2/15-300, L2/15-343, and L2/16-188) were discussed at WG2 #64, UTC #145, UTC #146, UTC #147, and UTC #148. The number of characters was finally reduced to 285, along with one that was unified with U+1B001 HIRAGANA LETTER ARCHAIC YE, which were accepted during WG2 #65 per Recommendation M65.03. The same set of hentaigana were accepted during UTC #149 for inclusion in Unicode Version 10.0, which is scheduled for a June 2017 release. A new block, named Kana Extended-A (U+1B100 through U+1B12F), was established to accommodate the 31 characters that could not fit in the existing Kana Supplement block (U+1B000 through U+1B0FF).
The figure below illustrates how hentaigana are associated with parent ideographs and with syllables. Multiple hentaigana sometimes correspond to the same syllable or were derived from the same parent ideograph. Hentaigana that were derived from the same parent ideograph sometimes correspond to multiple syllables.
I was very pleased to play a small role in the process that ultimately led to the encoding of hentaigana in Unicode. The UTC‘s involvement also helped to accelerate the process thanks to its four meetings per year.