The vassa retreat has largely been given up by Mahayana Buddhists, as Mahayana Buddhism has typically flourished in regions without a rainy season. However for Mahayana schools such as Zen and Tibetan Buddhism other forms of retreat are common.
Yawares, this statement is partially true.
Whilst there are retreats of all sorts done in Mahayana & the sister Vajrayana Tradition, its monastic adherents maintain their Vassa too.
a. In 'Mahayana Buddhism', there is no 'Vinaya' per se. It is derived from two main sources: the earlier various Indian Sravaka Schools and the Bodhisattva Vows (what some Mahayanists in limited circles regard as a 'higher Vinaya' for ordination) and is followed mainly by the East Asian Mahayana Tradition and the sister branch Vajrayana (e.g Tibetan, Nepalese, Himalayan and Japanese streams) in their countries of origin and overseas transmissions.
Today's East Asian Mahayana have monastic ordination from the ancient Indian Sravaka School known as the Dharmaguptaka, which is considered as a 'close cousin' to Theravada and the Tibetan, Nepalese & Himalayan Vajrayana streams follows another 'close cousin' known as the Mulasarvastivada. Plus Theravada's Pali Vinaya, these other two Vinayas have survived until today and used for monastic ordinations. In Tibet's case, if the king back then during the great Indian Master Atisa Dipamkara's time would have allowed it, his own ordination Vinaya, the Mahasanghika would be a fourth one today but for reasons of uniformity and preventing ordination confusion, the Mulasarvastivada Vinaya was upheld as introduced and transmitted earlier by another prominent pioneer Indian Master to Tibet, Santaraksita and thus Atisa did not conduct any ordination as per the king's wishes.
b. In East Asian Mahayana (Chinese, Korean and Vietnamese Traditions with the exception for the Japanese) which follows the Dharmaguptaka Vinaya, they observe the annual Varsah (Vassa) which traditionally begins on the 15th Day of the 4th Lunar Month (coinciding with Vesak Day) and ends around the 15th Day of the 7th Month, hence the famous occasion of the Ullambana celebration is joined with the Pravarana (Pavarana) Day, marking the end of this 'Rains Retreat' or also known as 'Summer Retreat' for them and the laity would come forth with the offerings of the requisites for the Sangha and other customary devotional practices. There are some variation in the dates of observance due to lunar calendar calculations but it is around this period of time. The same for Tibetan Vajrayana Tradition which also have monastics who observe it but I am unsure of the dates.
There was a case reported by the Chinese dailies in the northern part of my country where an elder bhikshu in the Chinese Mahayana Tradition, who migrated from Fujian, China, was ordained as a young child in the olden days of China and settled in my country during the World War II, who passed away some years back in his late eighties who clocked in the most number of Rains Retreat in my country, some say 60-70 over retreats in his lifetime.
c. Remember the Bodhisattva Vows, 'what some Mahayanists in limited circles regard as a 'higher Vinaya' for ordination'? In the Japanese's case, although they had the Dharmaguptaka Vinaya introduced and transmitted to them earlier, the Buddhist Traditions (e.g Tendai, Zen, Shingon, Jodo Shu/Shinshu, Nichiren et al here in Japan and their overseas transmissions) here have decided to abandon it in favor of the Bodhisattva Vows for ordination instead for various reasons. So, although they wear robes, live in a seemingly monastic like community, taking up celibacy and so forth, they are not regarded as Bhikshu/Bhikshuni as their ordination source is not from the Vinaya. Mostly, their clerics are known as priests/priestess and are addressed as 'Reverend' and so forth and since the Bodhisattva Vows do not entail compulsory celibacy for its upholders, the phenomena of some getting married with children is not unusual. It is claimed that there are still very few today in Japan who still observe the Vinaya, alongside with their brethren from other Buddhist Traditions which have taken root in Japan like the Chinese Mahayana, Theravada & Tibetan Vajrayana in the last recent developments.